The invention concerns a method for adjusting the desired shim coil current values of a, in particular, tesseral-harmonic (offaxis) shim system to produce, in particular in the vicinity of the center of the shim system, a homogeneous magnetic field.
Such a procedure is known in the art from the laid open publications DE 35 08 332 A1, EP 0 219 013 A2 and the Journal Article "The Ancient and Honorable Art of Shimming", G.N. Chmurny and D.I. Hoult, Concepts in Magnet Resonance, 1990, 2, 131-149. A1 The laid open publication DE 35 08 332 A1 describes a procedure and an apparatus for the production of particularly homogeneous magnetic fields with which the distribution of the main field which is to be rendered homogeneous is measured at a plurality of points so that an optimum magnetic field can be determined in a simple and reliable fashion, the field being generated via computer using a correction magnetic field production apparatus, the correction magnetic field being utilized for optimizing and producing the particularly homogeneous magnetic field. The invention utilizes the method of smallest quadratics in order to determine optimal values of the correction magnetic field. Initially, a cube with a side length of 0.3 meters is taken for the chosen region wherein the magnetic field is supposed to be homogeneous and the main field distribution is recorded in a plurality of characteristic planes, by way of example z=0.15 m, z=0, z=-0.15 m at e.g. 343 measurement points within the cube. In this manner, the 3-dimensional field distribution of the main field which is to be rendered homogeneous is measured at predetermined coordinates via a determination of a plurality of local main field values. The field distribution obtained in this manner is represented by a polynomial which includes a spatially unchanging component, a spatially changing component and a negligibly small residual component and, using the terms of the spatially changing components of the main field, the regulating current through the correcting coils with which the corresponding inhomogeneities are compensated is calculated. The purpose of this invention is to present a method and an apparatus for the production of particularly homogeneous magnetic fields which allows for an improved and more rapid homogenization of the magnetic fields than was previously possible.
EP 0 291 013 A2 describes a method for the minimization of the inhomogeneities of a static bain field through the utilization of a plurality n of shim coils. The method is carried out in such a manner that, initially, the magnitude of the main magnet field is measured without shim fields at a plurality of locations on the surface of an imaginary sphere. Subsequently, the magnitude and the polarity of each of n shim fields is measured as a function of the magnitude and polarity of a DC current flowing through the corresponding shim coil at the same locations on the imaginary spherical surface. Subsequently, an average quadratic error of the total field from the homogeneous field is determined at a plurality of other points within the volume of the imaginary sphere. Finally, the magnitude and the polarity of each current of the n-fold shim coil system is so adjusted that the previously determined error is minimized. The method assumes that the magnet field is expanded in a series expansion of spherical harmonics and that the corresponding correction coils produce partial fields which corresponding to spherical harmonic polynomials. The measurements on the surface of the imaginary sphere are carried out at points where the corresponding spherical harmonics have zeros to, in this matter, determine the strength of the current through the corresponding shim coil.
In the journal article "The Ancient and Honorable Art of Shimming", G.N. Chmurny and D.I. Hoult, a method is presented for the shimming of, in particular, a superconducting magnet for high resolution nuclear magnetic resonance. In contrast to the above-mentioned publications, where a plurality of measuring values of the main field with and without switching on of the shim coils is initially obtained and subsequently, using these obtained measuring points, the desired nominal current value of the shim coils is calculated, Chmurny and Hoult examine the influence of the various field inhomogeneities on the line width or the oscillating nuclear resonance signal of a sample excited by nuclear resonance. By means of iterative steps and by determination of the influence of the various shim coils on the nuclear resonance signal as a function of their current values, an optimal adjustment of the shim coils is achieved.
The above mentioned methods in accordance with the laid open publications DE 35 08 332 A1 and EP 0 219 013 A2 have the disadvantage that a large number of measuring points must be obtained under certain geometric relationships to another, resulting in a time consuming method and a complicated mechanical apparatus for the adjustment of the various measuring points. Furthermore, they require a fitting algorithm in order to produce the necessary comparison between the measurement data and the homogeneity condition which, in turn, does not always lead to an optimal solution. The Chmurny-and-Hoult-method has the disadvantage that a systematic method which, even when used by an inexperienced operator or technician would lead to a reliable and reproducible result, is not proposed which in turn, leads to results which depend strongly on the experience and the abilities of the individual who carries out the method. In addition, the result is not always achieved within tolerable homogeneity limits and/or the available adjustment time.
Therefore, it is the purpose of the present invention to produce a method for the adjustment of the current values of shim coils of a, in particular, tesseral-harmonic (off-axis) shim system which creates a simple systematic and reliable method which can even be carried out by an inexperienced technician to achieve reproducible and reliable results with which a minimum number of measurement points are necessary and a fitting algorithm is avoided.